UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

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The horse Silver dilution gene dilutes black pigment but has no effect on red pigment. The mane and tail are lightened to flaxen or silver gray, and may darken on some horses as they age. A solid black horse with this gene will be chocolate colored with a lightened mane and tail. A bay horse will have the black pigment on the lower legs, mane and tail lightened. Sometimes bay horses with Silver dilution can be mistaken for chestnuts with a flaxen mane and tail. Silver dilution is inherited as a dominant trait. It is known to occur in Rocky Mountain horses and related breeds, Shetland ponies, Icelandic and Morgan horses.

The gene responsible for Silver dilution has been identified as PMEL17 with a mutation in exon 11 being responsible for the dilute phenotype described above. Research has also confirmed the Silver dilution mutation to be associated with Multiple Congenital Ocular Abnormalities syndrome (MCOA), a wide range of ocular defects occurring in the anterior and posterior segment of the eye. The severity of the syndrome is dosage related, thus horses with 1 copy of Silver have less severe signs than those with 2 copies of the mutation. To avoid producing offspring with severe MCOA, breeders should not breed 2 Silver dilute horses together.

Silver Dilution results are reported as:

N/N No copies of Silver dilution detected.
N/Z One copy of Silver dilution detected.
Z/Z Two copies of Silver dilution detected. Horse is expected to have MCOA abnormalities.

Brunberg E, Andersson L, Cothran G, Sandberg K, Mikko S and Lindgren G. 2006. A missense mutation in PMEL17 is associated with the Silver coat color in the horse. BMC Genetics 7:46

Andersson LS, Wilbe M, Viluma A, Cothran G, Ekesten B, Ewart S, and Lindgren G. 2013. EquineMultiple Congenital Ocular Anomalies and Silver Coat Colour Result from the Pleiotropic Effects of Mutant PMEL PLoS One. 2013 Sep 23;8(9):e75639.

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